Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

An apparatus and method for ultrasonic non-destructive testing provides
an elongate strip of ultrasound transmissive material coupled at a
proximal end to an object under test. The elongate strip has a transverse
cross-section with a width and thickness giving an aspect ratio greater
than unity and matched to the ultrasonic transducer such that excitation
induces a substantially non-dispersive ultrasonic signal to propagate
along the elongate strip to the proximal end and to enter the object
under test. These non-dispersive pulses are particularly suited for
time-of-flight measurements, thickness measurements, crack measurements
and the like. The elongate strip helps to separate the transducer from a
potentially hostile environment associated with the object under test.
The elongate strip also has a large area of contact with the object under
test allowing efficient transmission of energy into the object under
test.

Claims:

1. Apparatus for ultrasonic non-destructive testing of a solid object,
the apparatus comprising: an elongate strip of ultrasound transmissive
material, said elongate strip having a transverse cross-section with a
width and a thickness giving an aspect ratio greater than unity, a
proximal end and a distal end; an ultrasonic transducer coupled to said
elongate strip and matched thereto such that excitation of said
ultrasonic transducer induces non-dispersive ultrasonic shear wave
signals to propagate in a propagation direction along said elongate strip
to said proximal end, said signals being formed of components of
different frequencies having wavelengths extending from λshort
to λlong and being polarized in a direction perpendicular to
said propagation direction; coupling means for coupling said proximal end
to a surface of a solid object, wherein said non-dispersive signals are
coupled into the solid object; and wherein said thickness is less than
2.5 λshort and said width is greater than 5 λlong.

2. Apparatus as claimed in claim 1, wherein said elongate strip is formed
of a material having a shear velocity Cs and a shear wavelength
λ, where λ=CS/F and F is the frequency corresponding to
λ.

3. Apparatus as claimed in claim 1, wherein said thickness is less than
λshort.

4. Apparatus as claimed in claim 1, wherein said elongate strip is formed
of material having a transmission mission velocity Cbar and a
transmission wavelength λbar where λbar=Cbar/F
and F is the frequency corresponding to λbar.

5. (canceled)

6. (canceled)

7. Apparatus as claimed in claim 1, wherein said ultrasonic transducer
excites substantially only a single mode of propagating guided wave.

8. Apparatus as claimed in claim 1, wherein said non-dispersive
ultrasonic signals spread substantially cylindrically from said proximal
end to an object under test.

9. Apparatus as claimed in claim 1, wherein said ultrasonic transducer is
coupled to said distal end.

10. (canceled)

11. (canceled)

12. Apparatus as claimed in claim 1, wherein said elongate strip is bent
around an axis that is substantially parallel to said width of said
elongate strip and substantially perpendicular to the propagation
direction.

13. (canceled)

14. Apparatus as claimed in claim 1, comprising an ultrasound receiver
operable to receive reflected ultrasound from said object under test
resulting from said non-dispersive ultrasonic signals entering an object
under test.

15. Apparatus as claimed in claim 14, wherein said ultrasound receiver
comprises one or more further elongate strips each coupled to said object
under test at a respective position to receive said reflected ultrasound
and having an ultrasonic transducer to detect said reflected ultrasound.

16. Apparatus as claimed in claim 15, wherein said one or more elongate
strips and said ultrasonic transducer also form said ultrasound receiver.

17. (canceled)

18. Apparatus as claimed in claim 1, including a solid object under test,
said proximal end being fixed to said solid object by one of: (i)
welding; (ii) brazing; (iii) soldering; or (iv) bonding.

19. Apparatus as claimed in claim 1, including a solid object under test,
said proximal end being clamped to said solid object.

20. Apparatus as claimed in claim 19, wherein ultrasound transmissive
couplant is disposed between said proximal end and said solid object.

21. Apparatus as claimed in claim 19, wherein a clamp clamps said
elongate strip to said solid object with an adjustable force.

22. Apparatus as claimed in claim 21, wherein said clamp is coupled to
said solid object by studs welded to said object under test.

23. Apparatus as claimed in claim 1, wherein said solid object is: (i) at
a temperature greater than 200.degree. C.; and (ii) subject to above
background levels of ionizing radiation.

24. (canceled)

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Continuation of U.S. application Ser. No.
12/891,231 filed Sep. 27, 2010, now pending, which is a Continuation of
U.S. application Ser. No. 12/092,630 filed May 5, 2008, now pending,
which is a 371 national phase of PCT/GB2006/03415 filed Sep. 14, 2006
which, in turn, claims priority to GB Application No. 0522572.7 filed
Nov. 4, 2005.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the invention

[0003] The present invention relates to an apparatus and method for
ultrasonic non-destructive testing.

[0004] 2. Description of the Prior Art

[0005] The use of ultrasonic signals in the non-destructive testing of
materials is known. Thickness measurements may be carried out by sending
ultrasonic signals into a test material and measuring their
time-of-flight across the sample. Defect monitoring may be performed by
sending ultrasonic signals into a test material and observing their
reflection from the structure of a defect. Typically, an ultrasonic
transducer is placed in direct contact with the object under test.
Transmitted ultrasonic signals are then received by the transmitting
transducer also acting as a receiving transducer, or a second receiving
transducer may be employed. Such procedures are straightforward in
non-hostile environments, but significant technical obstacles must be
overcome in order to operate such transducers in hostile (e.g. high
temperature) environments.

[0006] The development of ultrasonic transducers and their ancillary
components capable of withstanding high temperatures for extended periods
of time is challenging. Most transducer materials are adversely affected
by high temperatures and furthermore, resilient buffer amplifiers are
required to convert signals for transmission along coaxial cables, which
themselves must withstand the environment. Suitable connectors and power
supplies must also be provided.

[0007] An attractive alternative would be to use an acoustic waveguide
made from a material capable of withstanding the hostile environment to
transmit the ultrasonic signal into the test object from a transducer and
ancillary components located in a non-hostile region. The end of the
waveguide would be attached directly to the region of interest of the
test sample. The use of an intermediary waveguide, however, is not a
trivial task. Ultrasonic inspection typically employs high frequency
(>1 MHz) pulsed waveforms, which are not easily transmitted along a
long waveguide with high fidelity, due to dispersion, multiple modes and
attenuation. Additionally, both the transducers and the test sample must
be efficiently coupled to the waveguide to avoid prohibitively high
energy losses.

[0008] A major problem to be overcome is dispersion and the presence of
multiple modes. FIG. 1 of the accompanying drawings shows dispersion
curves for a cylindrical rod waveguide. Some spread in the energy of the
transmitted signal is unavoidable, so for example a signal generated at a
centre frequency of 2 MHz will typically have energy between 1 MHz and 3
MHz. Hence, since the accurate identification and timing of ultrasonic
signals coming from the test sample is paramount to the non-destructive
testing procedures described above, it is highly desirable to transmit a
signal which is largely non-dispersive, i.e. its velocity is almost
constant with frequency, and is dominated by a single mode.

[0009] Dispersion in a waveguide and the possible modes are largely a
function of the product of the frequency of the signal and the smallest
dimension of the waveguide. Furthermore, in order to obtain good accuracy
for ultrasonic thickness gauging it is generally necessary to operate at
above 1 MHz. However, at higher frequency-dimension products more higher
order modes may propagate and thus it is necessary to limit the smallest
dimension of the waveguide. Accordingly, the use of thin rod waveguides
is known in the art. Such devices are not without their own difficulties
though, since it is difficult to transfer sufficient energy into the thin
rod to produce a strong signal. Also, when a thin waveguide is joined to
a larger structure there is a strong surface reflection and relatively
little energy enters the structure. Additionally, a thin rod waveguide
coupled to the surface of a structure effectively acts as a point source,
from which energy spreads spherically, meaning that little energy returns
to the receiving waveguide, even from a strong reflector, such as the
bottom surface of the structure.

[0010] U.S. Pat. No. 5,962,790 (for example--see Refs 1, 2 and 3 and also
Ref 4) discloses a system using thin wire to minimize dispersion and
overcoming some of the problems of a single thin wire by employing a
bundle of thin wires. Each wire operates at a suitably low
frequency-diameter product, yet significantly more energy may be
transmitted through the multiple parallel wires in the bundle than
through a single wire. Nevertheless, bundles of wires are relatively
expensive to produce and become rather inflexible as their diameter
increases, limiting the geometries in which they may be deployed.
Furthermore, cross-talk between individual wires may complicate the
signal analysis and there are practical difficulties associated with
either attaching each individual wire to the test structure, or
terminating the bundle with a plate which does not introduce dispersion
problems. In terms of mode excitation, either extensional modes or a
torsional mode may be excited in a single wire. A torsional mode is
usually excited by a transducer in contact with the side of the wire, or
by an encircling electromagnetic coil. Such techniques are not practical
for a bundle of wires, where realistically only extensional modes may be
used.

[0011] U.S. Pat. No. 6,400,648 (Ref 5) discloses a coiled foil waveguide
as an alternative to a bundle of rods. The thickness of the foil is
arranged to be much smaller than the smallest wavelength of the
propagated signal, satisfying the low frequency-dimension product for
non-dispersive transmission. The foil is coiled around an axisparallel to
the direction of signal propagation, so if unwrapped would be very long
in a direction perpendicular to the direction of signal propagation.
However as the diameter of the coil increases, the waveguide becomes
rigid and damping due to rubbing between the layers may occur. Like a
bundle of wires, a coiled foil is better suited to extensional rather
than torsional waves.

[0012] U.S. Pat. No. 5,828,274 (Ref 6) discloses a tapered ultrasonic
waveguide with an external layer of attenuative cladding. The cladding
removes the effects of the waveguide boundaries by damping and limiting
surface reflections. This has the effect of removing almost all trailing
echoes, however the effects of dispersion are not entirely removed and
the signal is slightly delayed, slightly distorted and strongly
attenuated. The latter disadvantage limits the length of such a
waveguide, which is also rather inflexible. This is an improvement over
previous proposals using non-uniform threaded bars as waveguides (see
Refs 7 and 8).

[0013] U.S. Pat. No. 6,047,602 discloses an ultrasonic waveguide for fluid
flow metering which is a rectangular cross sectioned bar with an angled
end section. A surface of the angled section reflects energy travelling
along the bar into a narrow directed beam to enter the test fluid. The
waveguide is designed to maximize the energy transfer across a conduit.
This device has significant disadvantages in the field of thickness
measurement or defect monitoring, being inflexible and the wave
propagation not being optimized for a clean undistorted signal shape,
which is of utmost importance for timing measurements in the
non-destructive inspection of a sample.

[0014] This is a technical problem of providing a practical apparatus for
ultrasonic non-destructive testing capable of operating in hostile
environments and addressing the above described problems.

SUMMARY OF THE INVENTION

[0015] According to the present invention there is provided an apparatus
for ultrasonic non-destructive testing of an object under test, said
apparatus comprising:

[0016] an elongate strip of ultrasound
transmissive material, said elongate strip having a proximal end for
coupling to said object under test and a distal end; and

[0017] an
ultrasonic transducer coupled to said elongate strip; wherein said
elongate strip has a transverse cross-section with a width and a
thickness giving an aspect ratio greater than unity and matched with said
ultrasonic transducer such that excitation of said ultrasonic transducer
induces substantially non-dispersive ultrasonic signals to propagate
along said elongate strip to said proximal end and to enter said object
under test.

[0018] The present invention recognizes that there is a need for
ultrasound signals for non-destructive purposes to be transmitted in a
substantially non-dispersive manner, such that precise timing
measurements may be made. The present invention further recognizes that
there is a need for an ultrasound transmission component to be flexible,
such that the apparatus may be deployed in awkward geometries.
Accordingly, by transmitting ultrasonic signals along an elongate strip
with a width and thickness aspect ratio greater than unity and by
exciting signals which are substantially non-dispersive, ultrasonic
non-destructive testing of a test object may be performed in environments
hostile to traditional ultrasound transducers and in configurations which
require the transmission component to flexibly circumvent intervening
objects.

[0019] The elongate strip is formed of a material having a shear velocity
CS and a shear wavelength λB, where
λB=Cs/F and F is the frequency corresponding to
λB, and said substantially non-dispersive ultrasonic signals
are formed of components of different frequencies and having shear
wavelengths extending from λShort to λLong. Some
shear modes of ultrasonic waves are advantageously non-dispersive and
have the shortest wavelengths. Shorter wavelengths provide finer spatial
resolution for inspection purposes.

[0020] In a similar manner said elongate strip is formed of a material
having a bar velocity Cbar and a bar wavelength λbar,
where λbar=Cbar/F and F is the frequency corresponding to
λbar, and said substantially non-dispersive ultrasonic signals are
formed of components of different frequencies and having bar wavelengths
extending from λshort to λlong. Compressional waves
may be better suited to some situations.

[0021] Whilst the dimensions of the elongate strip can vary considerably,
in preferred embodiments, the thickness of the elongate strip will be
less than 2.5 times λshort. In particularly preferred
embodiments the thickness of the elongate strip will be less than
λshort. Such dimensional limitations help avoid the excitation
of undesirable higher order modes.

[0022] In preferred embodiments said width is greater than 3.5
λlong. In still more highly preferred embodiments said width
is greater than 5 λlong. Such dimensional limitations help
provide that the ultrasonic wave propagation is substantially
non-dispersive, has low amplitude at the waveguide edges and that the
mode shape is approximately constant.

[0023] Whilst differing ultrasonic modes may be used in preferred
embodiments, said substantially non-dispersive ultrasonic signals
comprise lowest order shear mode vibrations with a polarisation
perpendicular to the propagation direction and parallel to said width.
Such signals may be transmitted with low distortion and high efficiency
along a waveguide as described above.

[0024] In other embodiments, said substantially non-dispersive ultrasonic
signals comprise lowest order compression mode vibrations with
polarisation parallel to the propagation direction. The use of such waves
is beneficial in applications where high shear wave attenuation is
encountered in the test object or where the use of compressional waves in
the test object is advantageous.

[0025] Whilst the simultaneous excitation of multiple modes of a suitable
type is possible, in preferred embodiments, said ultrasonic transducer
excites substantially only a single mode of propagated guided wave. In
other embodiments said ultrasonic transducer is apodised to excite
substantially only said single mode of propagating guided wave to induce
said substantially non-dispersive ultrasonic signals. The restriction to
a single mode is advantageous in applications which require precise
timing information, since separately received signals can more readily be
identified as coming from separate features in the test object rather
than being the result of different modes.

[0026] Relative to the desired propagation distances in the object under
test, in preferred embodiments, said substantially non-dispersive
ultrasonic signals spread substantially cylindrically from said proximal
end to said object under test. The decay rate of the amplitude of a
cylindrically spreading wave is proportional to the reciprocal of the
square root of the distance from the source, whereas the amplitude of a
spherically spreading wave is proportional to the reciprocal of the
distance from the source. The former propagation therefore loses less
energy.

[0027] Whilst the transducer could be attached to a variety of positions
on the elongate strip, in preferred embodiments, said ultrasonic
transducer may advantageously be coupled to said distal end. In this
context said ultrasonic transducer is coupled to said distal end by one
of:

[0028] (i) a bonded connection;

[0029] (ii) a mechanical fixing and
ultrasound transmissive couplant; and

[0030] (iii) a mechanical fixing
and variable force.

[0031] Such couplings between the ultrasonic transducer and the distal end
of the waveguide promote efficient energy transfer between the two.
Bonding includes welding and brazing as well as other bonding techniques.

[0032] The coupling of the transducer to the elongate strip can be
achieved in a number of different ways. In one preferred embodiment said
ultrasonic transducer comprises a transducer coupled to at least one
longitudinal side of said elongate strip. In another preferred embodiment
said ultrasonic transducer comprises a coil operable to provide
electromagnetic ultrasound transduction. Such arrangements allow
alternative efficient methods of exciting ultrasonic signals in the
waveguide.

[0033] In one embodiment of the present invention said elongate strip is
bent around an axis that is substantially parallel to said width of said
elongate strip and substantially perpendicular to the propagation
direction. This allows particularly easy routing of the ultrasonic
signals in confined real life situations.

[0034] The receipt of the ultrasonic signal can be provided in a variety
of different manners. In one preferred embodiment, said apparatus
comprises an ultrasound receiver operable to receive reflected ultrasound
from said object under test resulting from said substantially
non-dispersive ultrasonic signals entering said object under test. In
this context, said ultrasound receiver comprises one or more further
elongate strips each coupled to said object under test at a respective
position to receive said reflected ultrasound and having a receiving
ultrasonic transducer to detect said reflected ultrasound. In another
preferred embodiment, said elongate strip and said ultrasonic transducer
also form said ultrasound receiver.

[0035] Whilst a variety of ultrasonic testing methodologies may be used in
conjunction with the present technique, in a preferred embodiment, said
reflected ultrasound comprises at least one reflected signals and said
ultrasound receiver measures a time difference between said reflected
signals. Such a time difference measurement gives information about the
structure of the test object.

[0036] The present invention recognizes that whilst the coupling of said
elongate strip to said object under test can be achieved in many
different ways, it is important to the performance of said apparatus and
in a preferred embodiment, said proximal end is fixed to said object
under test by one of:

[0037] (i) welding;

[0038] (ii) brazing;

[0039]
(iii) soldering; and

[0040] (iv) bonding.

[0041] In another preferred embodiment, said proximal end is clamped to
said object under test. In this context, the coupling may be improved by
ultrasound transmissive couplant being disposed between said proximal end
and said object under test. Furthermore, improved coupling may be
achieved in an embodiment wherein a clamp clamps said elongate strip to
said object under test with an adjustable force. In preferred embodiments
said clamp is coupled to said object under test by studs welded to said
object under test. Another preferred form of clamp is one that is wrapped
around the object under test, e.g. around a pipe.

[0042] The present technique is particularly useful when said ultrasonic
non-destructive testing is thickness measurement or crack monitoring.

[0043] The present techniques are particularly well suited to situations
in which said object under test is:

[0044] (i) at a temperature greater
than 200° C.; and

[0045] (ii) subject to above background levels
of ionising radiation.

[0046] Viewed from a second aspect the present invention provides a method
of ultrasonic non-destructive testing of an object under test, said
method comprising:

[0047] coupling a proximal end of an elongate strip to
an object under test;

[0048] exciting substantially non-dispersive
ultrasonic signals within said elongate strip to propagate along said
elongate strip and to enter said object under test.

[0049] Embodiments of the invention will now be described, by way of
example only, with reference to the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0050]FIG. 1 illustrates the phase velocity dispersion curves for various
ultrasonic modes in a steel rod;

[0061]FIG. 12 illustrates a signal from a waveguide welded to a steel
plate (6 mm thick); and

[0062] FIGS. 13 and 14 schematically illustrates clamping configurations
for attaching a waveguide to a sample under test.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0063] The present technique uses a thin strip waveguide (an elongate
strip). The phase velocity dispersion curves for a plate are shown in
FIG. 2 as a function of the frequency-thickness product. Below 1.4 MHz-mm
only three modes can propagate: S0 (the lowest order compression wave
with polarisation parallel to propagation), A0 (the flexural wave at low
frequencies) and SH0 (the lowest order shear mode with polarisation
perpendicular to propagation and parallel to strip width). These waves
are analogous to the L(0,1), F(1,1) and T(0,1) modes in the rod
respectively. The A0 mode is highly dispersive and is not attractive for
testing purposes but the SO mode is minimally dispersive at low
frequencies while the SH0 mode is completely non-dispersive at all
frequencies. FIG. 2 shows that in the frequency range below 1.4 MHz-mm
the phase velocity of the SH0 mode is much lower than that of the S0
mode. Since the wavelength is given by the phase velocity divided by the
frequency it follows that at a given frequency the SH0 mode has a shorter
wavelength than the S0 mode. This often makes it more sensitive in
inspection applications. Preferred embodiments of the present technique
therefore use shear horizontal modes but there may be circumstances where
compressional modes like the S0 mode are preferable (for example in
applications where the shear wave attenuation is much higher than that of
the extensional wave so that the signals with the shear wave are too weak
to be used).

[0064] Use of the strip waveguide has the following advantages over the
single wire, bundle and coiled solutions proposed previously:

[0065] Since the cross sectional area is much higher than a single wire,
it is easier to obtain strong signals; also the reflection from the
interface between the waveguide and the structure is smaller so more
energy enters the test structure.

[0066] When the wave enters the test structure from the strip it tends to
spread cylindrically. This means that the wave amplitude in the structure
decreases at a rate proportional to 1/r due to beam spreading where r is
the distance from the attachment point. This compares with a decay rate
proportional to 1/r for a wire system where the beam spreading is
spherical.

[0067] The cylindrical beam spread pattern is very suitable for both
simple thickness gauging and crack sizing based on time of flight
diffraction (TOFD) (See FIG. 3 and FIG. 4).

[0068] It is easy to excite a shear wave or a longitudinal wave in the
strip by attaching a transducer to the end of the strip; it is also
possible to excite either type of wave by attaching appropriate
transducers to the sides of the strip if this is more convenient.

[0069] It is possible to weld, solder, braze or bond the wave guide to the
structure. It is also possible simply to use viscous ultrasonic gel
couplant at low temperatures and/or to tightly clamp the waveguide onto
the structure (e.g. by welded on threaded studs giving an adjustable
clamping force) which works at high and low temperatures. Since the
waveguide is thin it is relatively easy to ensure that the whole bottom
surface of the waveguide is attached to the structure, so improving
signal transmission. Clamping the waveguide to the structure can be
advantageous since it removes the drawback of undesirable geometric
distortions that are inevitably introduced by permanent joining
techniques. Adequate signal transmission can be achieved by clamping.

[0070] The strip waveguide is much more flexible in one direction than a
typical bundle so it is easier to access structures around corners.

[0071]FIG. 5 shows an example embodiment of the invention. FIG. 6 shows
signals received in the thickness gauging application of FIG. 3 where the
thickness can be obtained from the time between the top surface and
bottom surface reflections or between successive backwall echoes knowing
the speed of sound. The sample may be at high temperature, e.g.
>200° C., and/or subject to above background levels of ionising
radiation.

[0072] The thickness of the strip should generally be chosen so that the
product of the thickness and the maximum frequency excited is less than 3
MHz-mm for SH type waves and 1.4 MHz-mm for longitudinal type waves in
order to avoid the excitation of higher order modes across the thickness.
If a longitudinal wave is used it is also often desirable to use a lower
frequency-thickness product in order to minimize dispersion, though it is
also possible to compensate for dispersion (see Ref 10). The width of the
strip is also an important parameter.

[0073] When the elongate strip is formed of a material having a shear
velocity Cs, and a shear wavelength λB, where
λB=Cs/F and F is the frequency corresponding to
λB, and said substantially non-dispersive ultrasonic signals
are formed of components of different frequencies and having shear
wavelengths extending from λshort to λLong. It is
desirable, but not essential, that the thickness be less than 2.5
λshort and particularly preferred to be less than
λshort. Similarly, it is preferred that the width is greater
than 3.5 λLong and particularly preferred that it is greater
than 5 λLong. In a similar way the same preferred ranges apply
when said elongate strip is formed of a material having a bar velocity
Cbar and a bar wavelength λbar, where
λbar=Cbar/F and F is the frequency corresponding to
λbar, and said substantially non-dispersive ultrasonic signals
are formed of components of different frequencies and having bar
wavelengths extending from λshort to λlong.

[0074] The dispersion curves in FIG. 2 model the wave propagation in a
plate of infinite width. Structures with a very large width to thickness
ratio will be modelled extremely accurately by this infinite width
assumption. However the closer the side surfaces are moved together to
create a strip of rectangular cross- section the more the wave
propagation will be influenced by the presence of the boundaries of the
strip. Mindlin and Fox (see Ref 11) were the first to describe the
propagating modes of a bar of rectangular cross section. Their solution
was made up of a superposition of several flexural, longitudinal and
shear modes that propagate in an infinite plate of the width and
thickness of the bar respectively. The solutions for the infinite plate
were superposed in order to fulfill the boundary conditions of zero
stress all around the perimeter of the cross section. This method enabled
them to determine the dispersion characteristics of the bar at distinct
frequencies and aspect ratios of the bar. A solution for all frequencies
and aspect ratios was however not possible. More recently however the
continuous tracing of dispersion curves for wave propagation in
structures of arbitrary cross section has become possible through the use
of finite element (FE) eigensolvers. Wilcox et al. (see Ref 12), Mukdadi
et al. (see Ref 13) and Hayashi et al. (see Ref 14) have reported methods
of tracing dispersion curves for L-shaped sections, rail heads and
strips.

[0075] The method of Wilcox et al. has been employed here to analyse the
modes propagating in a 1 mm thick and 30 mm wide strip of steel. FIG. 7
shows the phase velocity dispersion curves. The mode highlighted by the
thicker line has been identified as the first shear horizontal mode SH*
of this strip. In contrast to the infinite plate case the propagating SH0
mode does not exist in a finite strip. This is due to the zero stress
boundary condition on the strip sides, which can only be satisfied by
rigid body motion or the SH* and higher order modes. As a consequence no
non-dispersive propagating shear mode exists in a strip of finite width.
However the thicker the strip, the lower the cut-off frequency of the SH*
mode becomes. Since the SH* mode asymptotes to the bulk shear velocity in
the material it becomes virtually non-dispersive at higher frequencies.
In this invention the use of pure virtually non-dispersive strip modes of
compressional or shear nature is used to convey ultrasonic energy along a
wave guide or `acoustic cable` to a specimen that is to be interrogated.
The purity of mode avoids the arrival of several signals that could be
mistaken for a defect or feature in the interrogated structure and the
non-dispersiveness of the mode helps to concentrate wave energy in a
narrow time window, which increases the propagation range of the signal
in the waveguide and also determines the spatial resolution with which
the structure can be monitored (see Ref 15). An example based on the SH*
mode will be elaborated here while the use of similar modes of
compressional nature is also possible.

[0076]FIG. 8 shows the cross section of a 15 mm wide strip and the mode
shape of the SH* mode at 2 MHz. It can be seen that the y displacements,
which are displacements parallel to the width direction of the strip,
dominate the mode shape of the SH* mode. However unlike the mode shapes
of SH modes in infinite plates the mode shapes of SH modes in a strip of
finite width change with frequency. FIG. 9 shows the evolution of the
dominant y displacement component of the SH* mode over a range of
frequencies. The higher the frequency the more the mode concentrates at
the centre of the strip. Thus at high frequencies the mode propagates
energy at almost the bulk shear velocity along the centre of the strip
with little energy and thus sensitivity at the edges.

[0077] By means of an apodized transducer, that mimics the mode shape of
the mode, pure mode shape excitation can be achieved. Coil transducers
may also be used. The transducers can be placed on the ends or the sides
of the strip. Since short temporal pulses are broadband signals the mode
shape of the mode should not change significantly over the frequency
bandwidth of the excitation pulse. At higher frequencies (around centre
frequency of 2 MHz) this is the case for the SH* mode of strips of larger
width. Therefore there are two criteria that govern the allowable range
of aspect ratios of the strip. The first is that the mode is virtually
non-dispersive (velocity variations of less than 5%) in the frequency
bandwidth of interest and the second is that a relatively constant mode
shape over the frequency bandwidth of interest exists (less than 10%
difference in normalized amplitudes). For the SH* mode it is now the task
to find the limit of aspect ratio for which both criteria are satisfied.

[0078] The dispersion curves for an infinite plate can be made geometry
independent by plotting the curves against the frequency thickness (FT)
product. Therefore the cut-off mode on a plate twice the thickness of
another plate will occur at half the frequency. Since the width/thickness
ratio is large (>5) in our case the system can approximately be scaled
by two products; the FT product for frequency-thickness of the strip and
the FW product for frequency-width of the strip. Reasoning along those
lines and keeping the thickness of the strip constant (FT constant), the
width of the strip can be adjusted up to a limiting case where the
dispersion curves and mode shapes do not fulfill the earlier stated
criteria for successful excitation and propagation of a single mode. FIG.
10 shows the SH* mode phase velocity dispersion curves for a 1 mm thick
and 30 mm wide strip and a 1 mm thick and 15 mm-wide strip. The cut-off
frequency of the mode in the 15 mm wide strip occurs at double the
frequency. FIG. 11 shows the mode shape of the SH* mode near the cut-off
frequency (point 1.) and at frequencies near the point where the phase
velocities start asymptoting (point 2.) towards the shear bulk velocity
of the strip. At cut-off the mode exhibits large displacements at the
edges of the strip. The displacements at the edges become negligible at
higher frequencies.

[0079] Useful frequencies for inspection range from 1 MHz-5 MHz. However
broadband excitation with pulses is commonly employed in defect and
thickness monitoring. This requires a bandwidth of half the centre
frequency below and half above the centre frequency of the signal. Thus
for a 1 MHz centre frequency pulse the waveguide must exhibit similar
non-dispersive wave propagation over the range of 0.5.-1.5 MHz. Similarly
for a 2 MHz centre frequency signal this range extends from 1 MHz-3 MHz.
As can be seen in FIG. 10, the 1 mm thick and 15 mm wide strip becomes
unsuitable for sending a 1 MHz pulse. The phase velocity of the SH* mode
has not yet asymptoted to the shear bulk velocity. The mode is still
relatively dispersive in the frequency range 500-800 kHz and its mode
shape (FIG. 11(b)) does not have negligible amplitudes (˜25%) at
the edges. It can also be seen in FIGS. 11(a) and (b) that until the mode
starts to asymptote to the shear wave velocity its mode shape is not
dominated by the component in the Y direction. However the same strip is
suitable for sending a signal at 2 MHz centre frequency. Within the
bandwidth of a 2 MHz signal (1-3 MHz) dispersion is very limited and the
mode shape approximately constant (.+-0.5%). Therefore for successful
wave propagation (in form of the SH* mode) along the strip the
frequency-width must be greater than 15 MHz mm. Expressed in a ratio of
width over waveguide material bulk shear wavelength the width must be
wider than 5 bulk shear wavelengths at the lowest frequency component
contained within the signal.

[0080] Compressional Waves

[0081] The same procedure as above can be carried out for compressional
type strip modes. Their use might be beneficial in cases where high shear
wave attenuation is encountered in the structure or when the use of
compressional waves in the structure is of advantage.

[0082] Attachment

[0083] The transmission of energy across the `joint` of the `acoustic
cable` (waveguide) to the structure that is to be monitored is also
important. The problem of normal point & line forces on an elastic half
space is well known as Lamb's problem and was first solved by Lamb (see
Ref 16). Other workers have presented solutions for similar problems with
different geometries and source polarizations. Miller and Pursey (see Ref
17) considered infinitely long strips and discs loading the surface
normally, tangentially and in torsion. Achenbach (see Ref 18) presented a
solution for an infinite line source of anti-plane shear on an elastic
half space, which is a simplified 2D solution of a SH* mode in a
waveguide entering a larger structure. In this case the anti-plane shear
wave excited in the half space radiates cylindrically from the source
into the structure. The excitation of a surface wave that occurs under
all other loading conditions does not occur in anti-plane shear loading.
This is desirable for defect monitoring or thickness gauging since it
produces less complicated signals by eliminating mode conversions. Also
the radiation patterns described by Miller and Pursey for other sources
are more complicated and always contain a surface wave that claims a
large fraction of the energy. A strip source for a compressional
waveguide is also beneficial compared to a point source since it reduces
spherical spreading to cylindrical spreading of the wave from the source.
Also the thicker the strip the less energy is converted into a surface
wave.

[0084] The method of joining the waveguide to the structure is important.
Methods like welding, brazing or soldering as well as clamping dry
contact under a large normal force are possible. An attachment by welding
or soldering often alters the geometry of entry of the waveguide into the
structure. It is probable that fillets, residues of filler metal and
flash are produced along the edge where the waveguide meets the
structure. It is difficult to remove the fillets or residues, since they
are very close to the structure and the waveguide. The large difference
in thickness between the thin waveguide and the structure also makes it
very difficult to weld or solder the strip onto the structure without
damaging it. Changes in geometry introduced by the joining techniques are
almost always of the magnitude of the thickness of the waveguide and of
the order of a wavelength (at the given frequencies) of a wave in the
material. They cause reverberations of the signal within the joint and
degrade the signal that returns to the transducer. FIG. 12 shows such a
signal of a waveguide welded to a 6 mm back-plate. An entry reflection
and a backwall echo are identifiable; however the signal is covered by a
large amount of noise that is due to reverberations in the joint. This
can mask weaker signals of features of the structure. Initially it was
thought that welding, brazing, soldering or bonding the waveguide to the
structure was the best solution to attach the waveguide. Undoubtedly it
is the solution that transmits a lot of energy into the structure and is
a good option in the pulse-echo mode (send receive on the same
transducer). Drawbacks with the permanent joining methods are the
contamination of the signal with large noise due to reverberation in the
joint. These drawbacks are difficult to remove, therefore in cases where
signal "clarity" is important a clamp on method is preferred. The clamped
method works in pitch catch mode (send on one transducer receive on
another). The signal is injected into the structure via one waveguide and
it is picked up by a waveguide that is place right next to the sending
waveguide. The signal in the sending strip in `pulse echo` mode is hardly
changed for a clamped or free waveguide. This is because the waveguide
and substructure are not in very good contact when they are clamped and
pressed together. The large entry reflection, ringing and the presence of
other modes (30 dB lower than the signal) completely mask the low
amplitude signal from the structure. This problem is overcome by working
in pitch catch mode and using another waveguide as pickup for the signal
that has been transmitted into the structure. The result is seen in FIG.
6. The pitch catch configuration has the advantage of only picking up the
energy that has been transmitted into the structure and therefore
reducing the dependence on the ratio of transmitted to reflected
amplitude at the joint of waveguide and structure.

[0085] FIG. 13 and FIG. 14 show sketches of possible clamping
configurations to a plate like structure and a pipe. In the case of a
plate like structure a feature will have to be attached onto the
structure that enables the clamped waveguide to be forced onto the
surface of the structure. If possible studs can be welded onto the base
plate and the waveguide clamp can be screwed onto these studs. There are
many more variations possible. FIG. 14 shows a completely detachable
solution where the clamped waveguide is attached to the pipe by means of
two pipe clamps. It is important to have a clamp that only touches the
edges of the waveguide so that it does not influence wave propagation at
the centre of the strip. Also a grip close to the end of the waveguide
avoids buckling of the thin waveguide when it is forced onto the surface.